If you spend more than half a year somewhere, you are probably tax resident there. That is the intuition behind the number most people have heard: 183 days.
The intuition is roughly right and the details are where it goes wrong — particularly for anyone who moves in the middle of a year, which is most people.
What the count is actually for
Tax residency decides which country gets to tax your worldwide income. It is not the same as citizenship, not the same as where you are registered, and not the same as where your employer is. Two countries can each conclude you are resident, which is why double-taxation treaties exist.
A day count is the most objective test available, so many systems lean on one. It is rarely the only test.
Three mistakes
1. Counting the wrong year
Most of the EU uses the calendar year, but not all of it, and a tax year that starts in April changes every number. If you move in September, the two halves of your year may fall into different tax years in each country, on different schedules.
2. Counting only whole days
Many systems count any day on which you were present, including arrival and departure days. Two travel days per trip, over a year of moving back and forth, is a meaningful pile of days that people routinely leave out.
3. Assuming under 183 means not resident
This is the expensive one. Falling under the threshold in the new country does not automatically make you non-resident in the old one, and several countries have secondary tests that catch you regardless: a permanent home available to you, your family's location, where your economic interests sit, or simply where you are registered.
You can be under 183 days in both countries and still be resident somewhere. The day count is a filter, not a verdict.
Why mid-year moves are the hard case
Move on 2 January and the arithmetic is nearly trivial. Move on 14 July and you have two partial years, potentially two filing obligations, possibly a split-year treatment that some countries offer and others do not, and a set of days either side that you have to be able to evidence.
Evidence is the part people are least prepared for. Two years later, a tax office asking you to substantiate where you were in a particular month is not satisfied by memory.
The boring thing that solves most of it
Keep a log. Every trip, arrival date, departure date, country. Start it the day you decide to move, not the day someone asks.
It takes seconds per trip and it is the difference between answering a query in ten minutes and reconstructing two years from boarding passes and card statements.
Moving Country includes a hundred-row trip log that counts days per country per tax year automatically and flags when either crosses the common threshold.
To be explicit
This is not tax advice, and it deliberately contains no country's rules. Thresholds, tests, tax-year dates and split-year treatments differ across all 27 member states and change. Use the count as a record and a warning light, then confirm your actual position with the tax authority or a qualified adviser.